Implement transformed axis from boost-histogram/hist. (#675)

* Implement transformed axis from boost-histogram/hist.

* Use the right filename.

src/uproot/writing/identify.py

@@ -649,7 +649,10 @@ def _fXbins_maybe_regular(axis, boost_histogram):
         else:
             return edges.astype(">f8")
     else:
-        if isinstance(axis, boost_histogram.axis.Regular):
+        if (
+            isinstance(axis, boost_histogram.axis.Regular)
+            and getattr(axis, "transform", None) is None
+        ):
             return numpy.array([], dtype=">f8")
         else:
             return axis.edges

tests/test_0651-implement-transformed-axis.py

@@ -0,0 +1,55 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/uproot4/blob/main/LICENSE
+
+import os
+
+import numpy as np
+import pytest
+import skhep_testdata
+
+import uproot
+
+hist = pytest.importorskip("hist")
+
+
+def test(tmp_path):
+    filename = os.path.join(tmp_path, "testy.root")
+
+    h_reg = hist.Hist(
+        hist.axis.Regular(19, 0.1, 1e4, transform=hist.axis.transform.log)
+    )
+    h_var = hist.Hist(hist.axis.Variable(np.logspace(-1, 4, 20)))
+    h_reg.fill([0.15, 1.5, 15, 150, 1500, 15000])
+    h_var.fill([0.15, 1.5, 15, 150, 1500, 15000])
+
+    expectation = [
+        0.10000000000000002,
+        0.1832980710832436,
+        0.3359818286283784,
+        0.6158482110660265,
+        1.128837891684689,
+        2.0691380811147897,
+        3.79269019073225,
+        6.951927961775609,
+        12.742749857031336,
+        23.357214690901213,
+        42.81332398719395,
+        78.47599703514618,
+        143.84498882876622,
+        263.6650898730361,
+        483.2930238571756,
+        885.8667904100829,
+        1623.7767391887219,
+        2976.35144163132,
+        5455.594781168521,
+        10000.00000000001,
+    ]
+
+    with uproot.writing.recreate(filename) as f:
+        f["h_reg"] = h_reg
+        f["h_var"] = h_var
+
+    with uproot.open(filename) as f:
+        print(f["h_reg"].to_hist().axes[0].edges.tolist())
+
+        assert f["h_reg"].to_hist().axes[0].edges.tolist() == pytest.approx(expectation)
+        assert f["h_var"].to_hist().axes[0].edges.tolist() == pytest.approx(expectation)